Raw materials of natural origin are potential sources of various fuels or fuel components. For example, tall oil, a by-product of the kraft pulping of coniferous trees, has been used as raw material for hydrocarbon fuel components. U.S. Pat. No. 5,705,722 describes converting unsaturated fatty acids of tall oil to cetane improvers for diesel fuels. According to this patent, a feedstock consisting of tall oil is fed through a catalytic reactor by contacting it simultaneously with gaseous hydrogen. The resulting product is drawn off the reaction as one product stream which is further fractionated by distillation, from which cetane stream is drawn off as middle distillate.
In the above-mentioned process, depitched tall oil is used as raw material feedstock. According to the document, depitched tall oil is obtained by evaporating crude tall oil, for example by thin-film evaporator, to remove unsaponifiables and ash in the tall oil, which is followed by possible further distillation stages for the fractionation of fatty acids, diterpenic acids etc. The depitching (thermal evaporation) reduces both unsaponifiables and ash in the tall oil. The desired properties of the finished depitched tall oil include light colour, low ash content and high acid number. The depitched tall oil used in the process contains about 5-20 wt % unsaponifiable components. The document mentions also that when making depitched tall oil, ash can be removed by washing the tall oil with water and unsaponifiables can be removed by solvent extraction, solid-phase adsorption or liquid chromatography.
Document EP-1728844 A1 describes a process for producing hydrocarbon fraction useful as diesel fuel from biorenewable feedstocks by pretreating the feedstock to remove contaminants, such as alkali metals, that can poison the downstream catalyst, and then subjecting the pretreated feedstock to catalytic hydrogenation and decarboxylation/hydrodeoxygenation steps. Crude tall oil is mentioned as one example of biorenewable feedstocks, besides many triglyceride-based vegetable oils. The pretreatment step can include either ion-exchange with an acidic ion exchange resin or washing with an acid. In the example submitted later in the files of this document, washing soybean oil with 40% phosphoric acid solution was presented as an example of acid wash.
Document WO-2008/058664 describes a process for producing hydrocarbon fractions, which comprises successive hydrodeoxygenation (HDO) and hydroisomerization steps of feedstock of biological origin, one example among many feasible raw materials being tall oil. Before the HDO step, the feedstock can be subjected to adsorption on a suitable material, ion exchange, or slightly acid washing using sulfuric acid, nitric acid or hydrochloric acid, to remove alkaline metals and earth alkaline metals (Na, K, Ca). Gaseous phase separated after the HDO step and containing hydrogen, water, CO, CO2 light hydrocarbons and possibly small quantities of H2S is subjected to purification by means of caustic washing or treatment with amines, such as monoethanolamine or diethanolamine, to obtain recyclable gaseous fraction consisting essentially of H2 and traces of CO.
Document EP 1741768 A1 describes a process for producing diesel range hydrocarbons from bio oils and fats comprising hydrotreating the feedstock in a hydrotreating step and isomerising it in an isomerisation step. A pretreatment step, degumming, is recommended for the feedstock, such as crude vegetable oil or animal fat, in order to remove phosphorus compounds, such as phospholipids. Degumming is performed by washing the feed at 90-105° C., 300−500 kPa(a), with H3PO4, NaOH and soft water and separating the formed gums. A major amount of metal components, which are harmful for the hydrotreatment catalyst, are also removed from the feedstock during the degumming stage. Degumming as described above is a standard procedure for removing phospholipids and metals from vegetable oils of natural origin based on triglycerides and containing significant amounts of gums, typically 0.5-3% by weight. Iron and also other metals may be present in these oils in the form of metal-phosphatide complexes.
Still one process using biological raw materials is described in European patent 1396531, where raw materials containing fatty acids and/or fatty acid esters, including tall oil, are converted to hydrocarbon components in catalytic hydrodeoxygenation and isomerization steps.
Crude tall oil (CTO) is a promising candidate as raw material for manufacturing various fuel components. In the solutions of prior art expensive distillation steps are used to produce tall oil fatty acid fractions from crude tall oil. These fatty acid fractions are then processed e.g. by catalytic HDO (hydrodeoxygenation) and isomerization to desired fuel components while the fuel potential of other fractions left out is lost. Conversion of crude tall oil to fuel components, especially diesel components, involves three basic problems, namely
1) quality of the CTO. CTO comprises impurities, such as residual metals (ash) and phosphorus, which cause poisoning of the catalysts used in the process. Also quality variations of the CTO cause problems in converting CTO to fuel components.
2) quality of the produced diesel product. With prior art methods it is difficult to produce fuel components from CTO that have low Cp, (cloud point) and high cetane number. It is also difficult to achieve good yield in these processes.
3) highly exothermic reactions in the catalytic hydrodeoxygenation (HDO) step, which cause deteriorating of the catalyst and shorten the catalyst life. This is often avoided by recirculating the HDO product stream, which causes more problems to the process. The product stream exiting from HDO namely comprises the impurities that the CTO feed had, when it was introduced to the HDO. Consequently, the impurities are enriched to the recirculated HDO product stream and the recirculation will increase the poisoning of the HDO catalyst.